Traditional interchip interconnect technologies, when deployed for terascale data storage and computing, face issues in transfer speed and energy consumption. The excessive ohmic loss and dispersion associated with copper interconnects in high performance electronic systems have led to a number of efforts focusing on characterization of the physical interconnects, high-speed drivers, and channel equalization in an attempt to mitigate these challenges. In addition, solutions to the board-level (i.e., 1-10 cm link length) and back plane-level (i.e., 10-100 cm link length) interconnects have garnered much attention around electro-optical solutions. These solutions suffer from integration issues surrounding laser sources, waveguides, and photonic devices with traditional silicon systems, as well as electrical-optical/optical-electric conversion and waveguide-chip interfacing issues associated with coupling power on- and off-chip.
A number of efforts have focused on all-electronic solutions to the short-range chip-to-chip communication problem, involving coupling a modulated mmWave or sub-mmWave carrier into a dielectric waveguide—so-called “radio over fiber” schemes. These concepts attempt to harness the wider available bandwidths at these higher frequencies, and require on- and off-chip apertures to radiate into polymer or silicon waveguides. These traditional schemes use lower carrier frequencies, leading to lower bandwidth and I/O density. A number of these works are based on off-chip components, which introduce integration challenges and do not readily lend themselves to higher frequency operation.
Techniques utilizing off-chip radiators, aside from increasing system integration complexity, inherently trade the original bandwidth distance constraint of copper interconnects in driving an off-chip coupler. This effect manifests itself as a decrease in coupling efficiency. In the case of a dual band coupler utilizing mode orthogonality, such systems have been demonstrated with a bandwidth of 35 GHz and coupling loss of 5 dB. A number of efforts have utilized die-to-package bond wires or patch antennas as radiators, coupling energy into plastic tube waveguides. For example, a traditional coupler was demonstrated with a bandwidth of 6 GHz and a coupling loss of 6 dB utilizing air core plastic tube waveguides. This approach presents a number of integration challenges in packaging, especially when high-density I/O integration is needed. Lastly, work has been done on utilizing integrated on-chip antennas to couple modulated carriers into waveguides. In another example, a traditional coupler with a bandwidth of 8 GHz was implemented using a micromachined silicon waveguide, exhibiting a coupling loss of 5.8 dB. While these efforts address the need for an on-chip coupler, they suffer from the well-known bandwidth-radiation efficiency tradeoffs associated with on-chip resonant antennas. This approach also requires the waveguide interface itself to be normal to the radiator surface to maximize the coupling efficiency.